This invention is generally directed to liquid developer compositions and, more specifically, the present invention relates to a liquid developer containing protonated ammonium AB diblock copolymers as charge directors. More specifically, the present invention relates to liquid developers comprised of a suitable carrier liquid and as a charge director certain ammonium AB diblock copolymers. The developers of the present invention can be selected for a number of known imaging systems, such as xerographic imaging and printing processes, wherein latent images are rendered visible with the liquid developer illustrated herein. The image quality, solid area coverage and resolution for developed images usually require, for example, sufficient toner particle electrophoretic mobility. The mobility for effective image development is primarily dependent on the imaging system used, and this electrophoretic mobility is directly proportional to the charge on the toner particles and inversely proportional to the viscosity of the liquid developer fluid. For example, a 10 to 30 percent change in fluid viscosity caused for instance by a 5.degree. to 15.degree. C. decrease in temperature could result in a decrease in image quality, poor, or unacceptable image development and undesirable background development, for example, because of a 5 percent to 23 percent decrease in electrophoretic mobility. Insufficient particle charge can also result in poor, or no transfer of the toner to paper or other substrates. Poor transfer, for example, can result in poor solid area coverage if insufficient toner is transferred to the final substrate and can also result in image defects such as smearing and hollowed fine features. Other problems, resolved, or substantially eliminated with the present invention and which can effect the performance and reliability of a liquid developer include 1) the inability to develop or transfer onto certain fluoroelastomer substrates, such as VITON.RTM. available from E. I. DuPont, due to a charge exchange reaction between the substrate and the liquid developer; and 2) an instantaneous decrease of from about 10 percent to 75 percent in the particle charge upon the addition of additional charge director material which is added into an operating imaging system to maintain the conductivity of the developer within preset bounds, usually between 5 and 20 ps/cm. To overcome or minimize such problems, the liquid toners of the present invention were arrived at after extensive research efforts, and which toners result in, for example, sufficient particle charge, generally corresponding to an ESA mobility greater than 1.5 E-10 m.sup.2 /Vs for excellent transfer and maintaining the mobility within the desired range of the particular imaging system employed. Advantages associated with the present invention include the use of certain economical ammonium AB diblock copolymers as charge directors that are superior to other known charge directors, like BASIC BARIUM PETRONATE.TM. and quaternized ammonium AB diblock copolymers or other AB diblock copolymers, such as those illustrated in U.S. Pat. No. 5,130,221, in that they improve the desired negative charge on the developer particles; in some instances the improvement, as measured by ESA mobility, is from -0.2 E-10 m.sup.2 /Vs when quaternized ammonium, AB diblock copolymers are used to - 2.0 E-10 m.sup.2 /Vs when protonated ammonium AB diblock copolymers of the present invention are selected. The greater toner charge results in improved toner transfer from the imaging member to a substrate like paper, transparency stock, or intermediate belt materials, such as VITON.RTM.; and improvement in some instances was observed to be from essentially no transfer to a transfer of 80 percent to 90 percent of the image. The aforementioned desired charge can result in improved image development and enhanced transfer, and improved charging after the addition of additional charge director material.
A latent electrostatic image can be developed with toner particles dispersed in an insulating nonpolar liquid. Such dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be generated by providing a photoconductive layer with a uniform electrostatic charge and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy. Other methods are also known for forming latent electrostatic images such as, for example, providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. After the latent image has been formed, the image is developed by colored toner particles dispersed in a nonpolar liquid. The image may then be transferred to a receiver sheet Also known are ionographic imaging systems.
Typical liquid developers can comprise a thermoplastic resin and a dispersant nonpolar liquid. Generally, a suitable colorant, such as a dye or pigment, is also present in the developer. The colored toner particles are dispersed in a nonpolar liquid which generally has a high volume resistivity in excess of 10.sup.9 ohm-centimeters, a low dielectric constant, for example below 3.0, and a high vapor pressure. Generally, the toner particles are less than 30 .mu.m (microns) average by area size as measured with the Malvern 3600E particle sizer.
Since the formation of proper images depends on the difference of the charge between the toner particles in the liquid developer and the latent electrostatic image to be developed, it is desirable to add a charge director compound and optional charge adjuvants which increase the magnitude of the charge, such as polyhydroxy compounds, amino alcohols, polybutylene succinimide compounds, aromatic hydrocarbons, metallic soaps, and the like, to the liquid developer comprising the thermoplastic resin, the nonpolar liquid and the colorant. A charge director can be of importance in controlling the charging properties of the toner to enable excellent quality images.
In U.S. Pat. No. 5,035,972, the disclosure of which is totally incorporated herein by reference, there are illustrated liquid developers with quaternized ammonium AB diblock copolymer charge directors, and wherein the nitrogen in the ionic A block is quaternized with an alkylating agent, while with the invention of the present application there are selected for the developers as charge directors ammonium AB diblock copolymers wherein an acid is utilized to form the ammonium ion, and wherein the resulting developers have the advantages as illustrated herein, including a less costly charge director preparation process since, for example, with the ammonium AB diblock copolymers the conversion with the acid (HBr) only requires about 3 to 5 hours at ambient temperature whereas for the quaternized ammonium AB diblock copolymer the same conversion with the alkylating agent (methyl bromide) requires about 16 to about 20 hours at 50.degree. to 60.degree. C.; thus the acid conversion is a less energetic process. Also, the material's cost is considerably less for the HBr conversion than for the methyl bromide conversion. Using the prices in an Aldrich Chemical Company catalog for the least expensive source of methyl bromide, which is 99.5+ percent methyl bromide gas, and the least expensive source of HBr, which is the 48 percent aqueous hydrobromic acid solution, the cost of the methyl bromide in the conversion to the quaternized AB diblock copolymer will be 8 times greater than the cost of the hydrobromic acid in the conversion to the ammonium AB diblock copolymer with the liquid toners of the present invention acceptable particle charge at lower charge director levels, improved toner development and excellent transfer efficiency of one from 15 percent to 79 percent, and resolution improvement of from essentially zero development to a solid area having a reflectance density 1.0 and resolution of 8.5 Ip/millimeter) can be achieved in embodiments.
U.S. Pat. No. 5,019,477 to Felder, the disclosure of which is hereby totally incorporated by reference, discloses a liquid electrostatic developer comprising a nonpolar liquid, thermoplastic resin particles, and a charge director. The ionic or zwitterionic charge directors may include both negative charge directors such as lecithin, oil-soluble petroleum sulfonate and alkyl succinimide, and positive charge directors such as cobalt and iron naphthenates. The thermoplastic resin particles can comprise a mixture of (1) a polyethylene homopolymer or a copolymer of (i) polyethylene and (ii) acrylic acid, methacrylic acid or alkyl esters thereof, wherein (ii) comprises 0.1 to 20 weight percent of the copolymer; and (2) a random copolymer of (iii) selected from the group consisting of vinyl toluene and styrene and (iv) selected from the group consisting of butadiene and acrylate. As the copolymer of polyethylene and methacrylic acid or methacrylic acid alkyl esters, NUCREL.RTM. may be selected.
U.S. Pat. No. 5,030,535 to Drappel et al. discloses a liquid developer composition comprising a liquid vehicle, a charge control additive and toner particles. The toner particles may contain pigment particles and a resin selected from the group consisting of polyolefins, halogenated polyolefins and mixtures thereof. The liquid developers are prepared by first dissolving the polymer resin in a liquid vehicle by heating at temperatures of from about 80.degree. C. to 120.degree. C., adding pigment to the hot polymer solution and attriting the mixture, and then cooling the mixture so that the polymer becomes insoluble in the liquid vehicle, thus forming an insoluble resin layer around the pigment particles.
U.S. Pat. Nos. 3,852,208 and 3,933,664, both to Nagashima et al., disclose colored, light-transparent photoconductive material which can be obtained by a condensation reaction of organic photoconductive substances with reactive colored components. The chemical combination of an organic photoconductive substance having at least one amino or hydroxyl group with a color development component having at least one active halogen atom produces the color developing organic photoconductive materials. Alternatively, the color developing materials can be obtained from the combination of an organic photoconductive substance having at least one active halogen atom with a color developing component having at least one amino or hydroxyl group. The color developing organic photoconductive material may be pulverized in a ball-mill, a roll-mill or an atomizer to produce a toner for use as a dry or wet developing agent, or may be used in combination with other colored substances or vehicle resins.
U.S. Pat. No. 4,524,119 to Luly et al. discloses electrophotographic dry development carriers for use with toner particles wherein the carrier core particles are coated with fluorinated carbon or a fluorinated carbon-containing resin. By varying the fluorine content of the fluorinated carbon, systematic uniform variation of the resistivity properties of the carrier is permitted. Suitable binders for use with the carrier core particles may be selected from known thermoplastics, including fluoropolymers.
U.S. Pat. No. 5,026,621 to Tsubuko et al. discloses a toner for electrophotography which comprises as main components a coloring component and a binder resin which is a block copolymer comprising a functional segment (A) consisting of at least one of a fluoroalkylacryl ester block unit or a fluoroalkyl methacryl ester block unit, and a compatible segment (B) consisting of a fluorine-free vinyl or olefin monomer block unit. The functional segment of block copolymer is oriented to the surface of the block polymer and the compatible segment thereof is oriented to be compatible with other resins and a coloring agent contained in the toner, so that the toner is provided with both liquid-repelling and solvent-soluble properties.
U.S. Pat. No. 4,139,483 to Williams et al. discloses a finely divided dry toner composition comprising a colorant, a thermoplastic resin, and a surface active additive which is capable of providing a desired polarity and magnitude of triboelectric charging potential to the toner composition. The surface active additives are selected from highly fluorinated materials.
U.S. Pat. No. 4,388,396 to Nishibayashi et al. discloses developer particles comprising pigment particles, a binder and an offset-preventing agent selected from the group consisting of aliphatic fluorocarbon compounds and fluorochlorocarbon compounds. Electrical conductivity can be imparted to the developer by causing electrically conductive fine particles to adhere to the surfaces of the particles.
Moreover, in U.S. Pat. No. 4,707,429 there are illustrated, for example, liquid developers with an aluminum stearate charge adjuvant. Liquid developers with charge directors are illustrated in U.S. Pat. No. 5,045,425. Also, stain elimination in consecutive colored liquid toners is illustrated in U.S. Pat. No. 5,069,995.
In copending patent application U.S. Ser. No. 986,316,the disclosure of which is totally incorporated herein by reference, there is illustrated a process for forming images which comprises (a) generating an electrostatic latent image; (b) contacting the latent image with a developer comprising a colorant and a substantial amount of a vehicle with a melting point of at least about 25.degree. C., said developer having a melting point of at least about 25.degree. C., said contact occurring while the developer is maintained at a temperature at or above its melting point, said developer having a viscosity of no more than about 500 centipoise and a resistivity of no less than about 10.sup.8 ohm-cm at the temperature maintained while the developer is in contact with the latent image; and (c) cooling the developed image to a temperature below its melting point subsequent to development.
In copending patent applications U.S. Ser. No. 009,202 (D/92570), now U.S. Pat. No. 5,306,591, and U.S. Ser. No. 009,192 (D/92571), now U.S. Pat. No. 5,308,731, the disclosures of which are totally incorporated herein by reference, there is illustrated a liquid developer comprised of thermoplastic resin particles, a charge director, and a charge adjuvant comprised of an imine bisquinone; and a liquid developer comprised of a liquid, thermoplastic resin particles, a nonpolar liquid soluble charge director, and a charge adjuvant comprised of a metal hydroxycarboxylic acid, respectively. The charge adjuvants and other appropriate components of these copending applications may be selected for the liquid toners of the present invention.
The disclosures of each of the patents and the copending patent applications are totally incorporated herein by reference.